Understanding substance use and incident HIV/STI among young black MSM Project Summary/Abstract Racial/ethnic disparities in HIV have been a hallmark of the US HIV epidemic for decades, with black/African Americans experiencing higher rates of HIV infection than white Americans. These disparities are also observed in men who have sex with men (MSM), among whom the highest incidence rates are in young black/African American MSM (YBAAMSM). Despite a rich literature demonstrating the prevalent levels of drug and alcohol use among MSM and the potential relationships between these substances and high-risk sexual behaviors, individual risk behaviors do not explain why black MSM and YBAAMSM are more likely to become infected with HIV, compared to other MSM. Studies of self-reported use have collectively demonstrated no greater substance use among black MSM than among white MSM. However, a key finding of our previous R01, which examined factors associated with prevalent and incident HIV infection among a prospective cohort of black and white MSM in Atlanta, was that black MSM had substantially lower validity of self-reported drug use, compared to white MSM. Biomarker-classified drug use was also more predictive of HIV seroconversions than self-reported sexual behavior measures, suggesting that, in this population, substance use may be a marker of high-risk partners and environments. Furthermore, the racial disparity in prevalent HIV infection was most associated with differences in network and community factors rather than individual factors. Despite emerging data on sexual partners and structures among YBAAMSM, little is known about the overlap of sexual and substance-use networks, though this might amplify HIV/STI transmission risk. Therefore, we propose to conduct a new prospective cohort study of 300 HIV-negative YBAAMSM in Atlanta, GA, to understand prospective patterns of biomarker-supported substance use in YBAAMSM, as well as its antecedents, contexts, and consequences. This new project will build on the strong institutional infrastructure, community partnerships, theoretical frameworks, analytical approaches, and multidisciplinary research that contributed to the success of our previous cohort study. Results will provide a more detailed understanding of the multilevel associations between substance use and HIV risk, and will yield new opportunities for combination interventions that address the syndemics of HIV/STI and drug use among YBAAMSM.